Clonazepam protein binding was investigated in sera from five different umbilical cords, 45 children (aged 2 mo to 12 yr), and five adults (aged 27 to 40 yr). The unbound fraction (means +/- SE) of clonazepam was 17.3% +/- 0.7% in umbilical cord serum and 13.9% +/- 0.2% in adult serum (P less than 0.01). In children, the unbound fraction of clonazepam reached the adult values during the first year of life. The kinetics of clonazepam serum protein binding were studied in three umbilical cord serum and three adult serum specimens. The number of binding sites (n, reported as moles per gram protein) and the association constant (K, reported as M-1) were estimated from double reciprocal plots of 1/r against 1/D (r is the number of moles bound per gram plasma protein; D is the molar concentration of unbound drug). In umbilical cord sera, the mean values (+/- SE) of n and K were 8.4 +/- 0.6 X 10(-7) mol/gm and 8.3 +/- 0.9 X 10(4) M-1. In adult serum samples the corresponding values were 3.0 +/- 0.8 X 10(-7) mol/gm and 2.7 +/- 0.6 X 10(5) M-1, which indicated lower binding capacity but higher affinity for clonazepam of plasma proteins in adults than in children.